The electrophotographic system is a type of image forming method in which generally, a surface of a photoconductor using a photoconductive material is charged, for example, by corona discharge in a dark place, exposure is performed thereto, the charge of an exposed portion is selectively allowed to dissipate to obtain an electrostatic latent image, and development is performed thereto using a toner, followed by transferring and fixing to paper or the like to obtain an image. An inorganic photoconductive material such as selenium, zinc oxide, cadmium sulfide or silicon has hitherto been widely used as an electrophotographic photoconductor. These inorganic materials have many merits and also have various demerits at the same time. For example, selenium has the disadvantages of requiring hard production conditions and of easily crystallizing by heat or a mechanical impact. Zinc oxide and cadmium sulfide have problems in moisture resistance and mechanical strength, and have the disadvantage in that deterioration of charging and exposure occurs by a dye added as a sensitizer, resulting in lacking in durability. Silicon also requires hard production conditions and is high in cost because a stimulative gas is used, and care must be taken in handling because it is sensitive to humidity. Further, selenium and cadmium sulfide also have the problem of toxicity.
Organic photoconductors using various organic compounds improved in the disadvantages of these inorganic photoconductors have been widely used. The organic photoconductors include a monolayer type photoconductor having a charge generating agent and a charge transporting agent dispersed in a binder resin and a laminate type photoconductor having a charge generating layer and a charge transporting layer functionally separated. The functionally separated type photoconductor has a wide option for each material, and a photoconductor having optional performance can be relatively easily prepared by combination, so that it has been widely used.
As the charge generating agents, many organic pigments and dyes such as an azo compound, a bisazo compound, a trisazo compound, a tetrakisazo compound, a thiapyrylium salt, a squarilium salt, an azurenium salt, a cyanine dye, a perylene compound, a non-metal or metal phthalocyanine compound, a polycyclic quinone compound, a thioindigo-based compound and a quinacridone-based compound have been proposed and put into practical use.
As the charge transporting agents, there are, for example, an oxadiazole compound (patent document 1), an oxazole compound (patent document 2), a pyrazoline compound (patent document 3), a hydrazone compound (patent documents 4 to 7), a diamine compound (patent document 8), a stilbene compound (patent documents 9 to 11), a butadiene compound (patent document 12) and the like. Organic photoconductors using these charge transporting agents have excellent characteristics, and some of them have been in practical use. However, no one sufficiently satisfying various characteristics required for photoconductors of the electrophotographic system has not been obtained yet under the present situation. Further, some are not put in practical use because of their poor compatibility with a resin or their poor solubility in a solvent, although various characteristics such as sensitivity are satisfactory.
In some patent documents published in the past, p-terphenyl compounds are used for applications of electrophotographic photoconductors. In patent document 13, compounds such as p-terphenyl compounds are disclosed, but these are contained in a charge generating layer of a laminate type photoconductor, thereby intending to improve electrophotographic characteristics such as durability and sensitivity. Further, p-terphenyl compounds disclosed in patent document 14 are excellent in solubility, but insufficient in various characteristics such as durability.
Patent Document 1: JP-B-34-5466
Patent Document 2: JP-A-56-123544
Patent Document 3: JP-B-52-41880
Patent Document 4: JP-B-55-42380
Patent Document 5: JP-B-61-40104
Patent Document 6: JP-B-62-35673
Patent Document 7: JP-B-63-35976
Patent Document 8: JP-B-58-32372
Patent Document 9: JP-B-63-18738
Patent Document 10: JP-B-63-19867
Patent Document 11: JP-B-3-39306
Patent Document 12: JP-A-62-30255
Patent Document 13: JP-A-61-129648
Patent Document 14: JP-B-6-73018